This invention relates to an alloy having a low melting point usable as a seal material for a metal-made vessel such as a vessel formed of stainless steel, and more particularly to a Bi-Sn-In alloy.
Conventionally, an alloy having a low melting point is employed as a seal material for use in a metal-made vessel such as a vessel formed of stainless steel. For example, a rotating plug 2 of a reactor vessel 1 shown in FIG. 1 is required to be sealed for purpose of preventing the leak-out into the atmosphere of a cover gas 4 having radio activity covering the upper surface of a coolant 3 within the reactor vessel 1. The sealing of the rotating plug 2 is effected by providing a circular blade 5 attached to an edge portion of the plug 2, respectively and immersing this blade within a trough 6 in which is received a fusible seal material 7 consisting of a low-melting alloy.
A Bi-Sn eutectic composition (Bi 57 weight %, Sn 43 weight %) is conventionally known as a seal material. This alloy has as high a melting point as about 140.degree. C., and simultaneously has no good sealing property. Further, a fusible seal material is also known whose melting point, i.e., solidification starting point is reduced to 100.degree. C. or less by adding a large amount of In to the Bi-Sn eutectic composition alloy. This alloy has Bi-Sn-In proportion of 60 to 64 weight %, 17 to 21 weight % and 17 to 21 weight %, respectively, and a solidification starting point of 79 to 89.degree. C., and has degraded sealing property and low oxidation resistance, and in addition uneconomically requires a large amount of In. Assume now that a Bi-Sn-In alloy having said proportion be used as a seal material for sealing the rotating plug 2 of the reactor vessel 1 shown in FIG. 1. Upon performing the rotation operation of the rotating plug 2, the seal material 7 is molten while during a normal operation of the reactor vessel the seal material 7 is solidified to fixedly hold the plug 2 in place. Since, in this case, the seal material 7 exhibits no sufficient degree of sealing property when having been solidified, a complete sealing of the plug 2 during the reactor vessel operation can not be expected. Further, during a period in which the seal material 7 is molten, that is, during the plug rotation, the seal material 7 is oxidized, for which reason the composition of the seal material is varied to decrease the reliability upon a condition in which the cover gas 4 within the reactor vessel 1 is sealed. Under such circumstances, there has been a demand for an inexpensive seal material having excellent sealing property and oxidation resistance.